1. Field of the Invention
This invention relates generally to signal generators. More particularly, this invention relates to signal generators which generate a signal of a frequency which is essentially constant over time.
2. Related Art
There are many technical applications in which constant frequency signal generation is required. Such applications include all forms of measuring and testing equipment, as well as clocks, computer and communications systems, navigational systems, and all forms of devices in which accurate measurement of periods of time is required.
Accurate measurement of time has been a technological goal for literally hundreds of years. In the recent past, substantial improvements have been made through the use of crystal oscillators. Even more recently, atomic clocks using the rate of electron transitions in atoms of selected elements, such as cesium, rubidium and hydrogen, have been increasingly common as sources of constant frequency signals. However, both quartz oscillators and atomic clocks suffer some substantial instability caused by, for example, temperature, magnetic influences, component aging, humidity, mechanical shock, etc.
There are a wide variety of uses for accurate oscillators, that is, signal generators which provide time signals which are constant. It has been said that nearly every electronic instrument contains an oscillator of some sort, and that oscillators are as important to such instruments as are stable sources of supply voltage. See generally Horowitz and Hill, The Art of Electronics (1980), generally at chapters 4 and 14, and particularly at .sctn.14.09, incorporated herein by reference. Note that the various oscillators described therein range greatly in accuracy and price.
For example, temperature-compensated crystal oscillators cost about $50 and are stable to several parts per million. Ovenized crystal oscillators range in price from several hundred to more than one thousand dollars, and have stability rates of parts per million, but are noted for their tendency to degrade with age, such that the frequency varies over time. They are large in size and are costly to calibrate and maintain.
Next on the scale of complexity and price are the atomic standards: rubidium, cesium and hydrogen. These are stated to have stability of up to one part in 10.sup.15, but are priced on the order of tens of thousands of dollars ($20,000 to $30,000 is typical). These also show degradation with time.